AQA GCSE Physics

Revision Notes

5.1.3 Forces as Vectors

Forces as Vectors

  • Forces are vector quantities as they are described by both magnitude and direction
    • The magnitude of a force is measured in Newtons
    • The direction of a force can be described as left, right, up, down or even using angles

Simple Force Vectors, downloadable IGCSE & GCSE Physics revision notes

A free body diagram of an object with two forces acting on it

  • The direction of a force can be imagined using a number line
    • Numbers to the left of zero are negative as are forces pointing left
    • Numbers to the right of zero are positive as are forces pointing right

Number Line, downloadable IGCSE & GCSE Physics revision notes

Vectors represented as positive or negative vectors depending on their direction

  • Not all forces are directed perfectly horizontally or vertically and thus need to have an angle described
    • It is useful to describe an angle with respect to the vertical or the horizontal

Force at Angle, downloadable IGCSE & GCSE Physics revision notes

A force of magnitude 100 N directed 40° to the horizontal

Force Pairs

  • When there is an interaction between two objects, a force is exerted on each object
    • This is known as a force pair
  • Examples of these force pairs include:
    • The downwards force due to the weight of a laptop resting on a desk, the desk exerts a normal force back up on the laptop
    • The force exerted by a basketball player throwing a basketball is opposed by the reaction force of the basketball on the player
    • The tension force exerted along a cable to a suspended object is opposed by the force of the weight of the object
  • Force pairs can be represented by arrows in vector diagrams

Force Pairs, downloadable IGCSE & GCSE Physics revision notes

The force pairs present with respect to a rock being pushed by a person

  • The applied force that the person exerts on the rock is opposed by a reaction force from the rock
  • The weight of the rock on the ground is opposed by a normal force
  • The weight of the person is also opposed by a normal force
  • The force applied by the person driving their feet into the ground is opposed by friction

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